Fiber optic cables are increasingly used in modem data and voice communications networks because of their small size and their ability to transmit a large volume of data and/or voice messages simultaneously. In any such network, a transmitted message may need to travel over a number of different connected fiber optic cables between the sender and the receiver of the message.
To accommodate the joining together of two fiber optic cables end to end in a communications network, a variety of cable connectors have been developed. One such connector is disclosed in U.S. Pat. No. 4,512,630 of P. K. Runge. In the Runge device, each of the optical cables to be joined is terminated in a plug having a truncated conical shape with a small diameter endface. The optical fiber of each of the cables is exposed on the endface of its respective plug. The cables are operatively joined together by being disposed in a bi-conical sleeve that holds the endfaces of the plugs together with the optical fibers aligned with each other.
Another fiber optic cable connector is AT&T's ST connector in which the end of each cable to be joined is terminated by a cylindrically shaped ceramic plug or ferrule having an axial passageway and a dome-shaped end. The optical fiber of each cable is secured within the passageway and extends along the axis of its respective ferrule and the endface of the fiber is exposed on the end of the ferrule. Traditionally, the ends of the ferrules are ground and polished until the endfaces of their respective optical fibers are smoothly polished and coplanar with or protrude slightly beyond the end of the ferrule. This configuration insures that, when the ends of two ferrules are brought together in the connector, the optical fibers align and contact each other so that light borne transmissions can cross from one fiber to the other.
Each ferrule in the ST connector is disposed in a respective cap and is yieldably biased to an outwardly protruding position by a compression spring in the cap. To connect the ends of two cables together, the caps are coupled to the opposite ends of a sleeve, which functions to align the ferrules axially and thus align the optical fibers exposed on the ends of the ferrules. As the caps are locked into the sleeve, the ferrule end and optical fiber endfaces engage and are depressed against the forces of their respective compression springs. The springs function to push and hold the ends of the ferrules, and thus the endfaces of the optical fibers, together with a predetermined force, which typically is the industry standard two (2) pounds for each compression spring, providing a total compressive force of four (4) pounds. The ends of the optical fibers are thus aligned and held together face-to-face to facilitate the transfer of a light borne message from one of the cables to the cable joined thereto.
While the specific connectors described above are commonly used in the industry and are used as examples in the present disclosure, it will be understood that a variety of connectors and connector mechanisms are commercially available. Such connectors take various physical forms, are made of various materials, and operate mechanically in various ways. However, such connectors all perform the basic function of aligning and holding together the polished endfaces of two fiber optic cables so that light borne data can be transmitted from one cable to the other. In some instances, attenuators or other devices are disposed between the ends of the cables within a connector depending upon the purpose of the connection.
The transmission of a message across the junction between two joined fiber optic cables is not a perfectly efficient process. In each case, a small amount of the light that carries the message is reflected back from the junction so that the transferred signal is weakened slightly. This phenomenon is known as "return loss" and can result in a variety of problems that range from a reduction in transmitted signal integrity to destabilizing effects on the back detectors that monitor and control the solid state lasers that create and insert the laser signals into the network.
Return Loss is specified and measured in decibels (dB) and is calculated as follows: ##EQU1## Where P.sub.reflected is the optical power reflected at the junction between two mated connectors and P.sub.incident is the optical power that enters the junction between the two connectors. For example, if the incident power is 1.times.10.sup.-3 watt and the flected power is 1.times.10.sup.-8 watt, then the return loss is computed as follows: ##EQU2## A return loss of 50 dB, for example, means that the portion of the light reflected back from a junction of two fiber optic cables is attenuated 50 dB relative to the portion that is transferred across the junction. Thus, a connector with a greater dB return loss rating transfers more signal from one cable to another and reflects less back as return loss.
While fiber optic cable connectors have steadily been improved over the years to provide greater return loss ratings, commercially available connectors seem to have reached a practical limit of between 55 and 57 dB. Connectors with greater dB loss ratings have been produced in the laboratory under carefully controlled conditions. However, such conditions have not lent themselves to commercial production techniques. Thus, manufacturers have been unable to produce, with commercial consistency, fiber optic cable connectors with return loss ratings better than about 57 dB, even though a rating of at least 60 dB would be highly superior and even though return losses as high as about 75 dB are theoretically possible.
The causes of return loss at the junction between two joined fiber optic cables are many. Among the causes are microscopic imperfections on and just below the surfaces of the polished ends of the optical fibers. These imperfections can range from surface scratches to subsurface fractures caused by the grinding and polishing process itself. Another cause of return loss arises from the fact that the ends of the joined optical fibers are pressed and held together within their connectors with a specified force, usually four (4) pounds. This force acts to compress the glass material of the fibers at their ends, creating an increasing molecular density gradient and thus an increasing optical index of refraction as light approaches the junction. The increased index of refraction in the region of the junction causes some of the light to be reflected back from the junction, resulting in return loss. Even though polishing techniques have improved, manufacturers have been unable to overcome these and other inherent causes of return loss and have thus been unable to produce fiber optic cable connectors with return loss ratings of 60 dB or greater in commercial quantities.
It should be noted that one type of known optical fiber connector, particularly, an angled physical contact (APC) connector, does routinely exhibit a return loss of greater than 60 dB. The APC connector has a beveled ferrule that is engaged and mated with a corresponding beveled ferrule of another APC connector. How ever, this type of connector is very difficult to produce and requires specialized manufacturing equipment, including specialized polishing equipment. Hence, for commercial mass production, the APC connector is not suitable.
Thus, there exists a need for an improved, commercially producible fiber optic cable connector that operatively joins the ends of two fiber optic cables together with a consistent causes loss of at least 60 dB. Such a connector should be compatible with existing connectors and should be producible with little or no increased production costs as compared to current connectors. The 60 dB return loss rating should be stable over time and should not be affected by temperature cycles to which such connectors are sometimes subjected. It is to the provision of such a fiber optic connector that the present invention is primarily direct.